Membrane Pore Morphology Research Articles

On the role of the porous support membrane in seawater reverse osmosis membrane synthesis, properties and performance

In this study, we explore the role of the polysulfone (PSU) support membrane skin-layer and whole-body pore morphology on the physical-chemical properties and separation performance of hand-cast polyamide-PSU (PA-PSU) composite seawater reverse osmosis (SWRO) membranes. We tailor the support membrane pore morphology by varying the PSU concentration and the coagulation bath temperature. In order to isolate the impacts of the PSU support, all of the porous supports are coated using identical m-phenylene diamine (MPD) and trimesoyl chloride (TMC) solution compositions, reaction conditions, and post-treatments. As PSU concentration increases, PSU support membrane pore size, surface and body porosity, water permeability, MPD mass uptake (by the PSU support), and PA-PSU composite membrane water permeability decrease, while MPD and TMC conversion, PA film mass and thickness, crosslinking degree (XD), and PA-PSU composite membrane NaCl rejection increase. As PSU support membrane coagulation bath temperature increases, support membrane pore size, surface and body porosity, MPD uptake, PA film mass, thickness and XD, MPD and TMC conversion, and NaCl rejection increase, while composite PA-PSU membrane water and NaCl permeability decrease. Composite membrane permeabilities were 70–90 percent of the (theoretical) unsupported PA film permeabilities due to the high XD and low PA coating film thickness. Composite PA-PSU membrane water/salt permeabilities both correlate most strongly with PA coating film mass/thickness and XD, which in turn correlate most strongly with TMC conversion. The key to increasing water permeability is lower PA film mass/thickness and XD with higher support membrane surface pore size and porosity. In contrast, the key to increasing NaCl rejection is higher PA film mass/thickness and XD with lower support membrane surface pore size and porosity.

Effects of nanosized organic‐rectorite fillers on the morphologies and properties of PVDF UF membrane

AbstractIn this study, polyvinylidene fluoride/organic rectorite (PVDF/OREC) organic–inorganic nanocomposite membrane was prepared by phase conversion method. TEM and X‐ray diffraction (XRD) results demonstrate the successful fabrication of nanocomposite ultrafiltration membranes. When the OREC is less than 3 wt%, the nanosheets are mainly stripped and dispersed in the polymer matrix, with different numbers of intercalations. When the impurity content is 5%, the intercalation distribution dominates. When the impurity content increases to 7% and above, it becomes increasingly difficult for PVDF chain segments to enter the OREC interlayer and the OREC particles are dominated by simple blends on the order of micrometers in the polymer matrix. The inclusion of inorganic fillings significantly improved the mechanical properties of the membrane. The yield strength and tensile modulus of the membrane were increased by 53.7% and 574.9%, respectively, with an inorganic filler content of 5 wt%, while the elongation at the break was slightly decreased to 16.2%. This suggests a strong interaction between the OREC nanosheets and the polymer matrix. XRD and FTIR analyses indicate that OREC particles are effective heterogeneous nucleating agents that favor the formation of β‐phase crystals in the film. SEM surface and cross‐section results show that when joining can significantly reduce the average pore size of the membrane surface, large holes tend to reduce the number of holes in the skin layer distribution more uniformly. The AFM results show that the addition of OREC can significantly alter the surface roughness of ultrafiltered membranes, and that the changes in surface roughness and surface pore morphology of membranes with different clay contents are closely related to the distribution of nanoscale silicate sheets in the PVDF matrix.Highlights Prepared PVDF/OREC nanocomposite membranes via a phase‐inversion process. OREC acted as a potent nucleation agent, favoring β crystal formation. OREC resulted in smaller pore sizes and fewer large pores in the skin layer. The hydrophilicity of PVDF membranes was reformative by filling OREC. OREC led to increased water flux and BSA rejection due to pores and hydrophilicity.

Impact of membrane pore morphology on multi-cycle fouling and cleaning of hydrophobic and hydrophilic membranes during MBR operation

This study investigated the impact of membrane pore morphology on repeated fouling and physical cleaning behavior during > 1000 h operation of a membrane bioreactor process. Four microfiltration membranes with different backbone polymers (PVDF and PTFE) and varied hydrophobicity, operated in parallel, were compared. The PVDF membranes presented a porous particulate-packing morphology, while the PTFE/PVDF blend membranes (PTFE accounting for > 80% of monomer proportion) resembled a nonwoven network of thin fibrous material. These membranes had similar short-term apparent fouling rates in a single fouling-and-cleaning cycle, but different long-term fouling development due to their physical cleaning efficiencies differing in the order: hydrophobic PVDF << hydrophobic blend < hydrophilic (PVDF and blend) membranes, as revealed by statistical analyses. The physically irreversible fouling was likely attributed to hydrophobic adsorption onto the membranes, with the adsorbed polysaccharides and proteins detected using confocal laser scanning microscopy. The irreversibility of fouling on the blend membranes was much less sensitive to membrane hydrophobicity than that on the PVDF membranes, probably because the blend membranes had smaller adsorption propensity. For further evidence, the adsorption properties of the membranes were explored via batch tests of dynamic adsorption using model compounds, with the adsorption rate and equilibrium constants quantified using the Thomas model. The thin thread-like pore walls (which characterized the morphology of the blend membranes) may provide limited contact area for foulant adsorption, and/or weaken the interfacial membrane–foulant interaction via shape effect. Therefore, it is suggested that membranes with thin fibrous network-like pore morphology may perform more robustly and be less troubled by adsorptive irreversible fouling for long-term operation of MBR.

Clay Nanoparticles Composite Membranes Prepared with Three Different Polymers: Performance Evaluation

This paper presents the results obtained from the evaluation of clay nanoparticles as an additive for improving the characteristics and performance of composite membranes cast with polysulfone (PS), polyethersulfone (PES), and polyvinylidene fluoride (PVDF). Different concentrations of clay nanoparticles, ranging from 1 to 10% based on the polymer mass, were used to prepare all dope solutions. The addition of clay nanoparticles changed the internal pore morphology of membranes, which resulted in significant changes on their performance, regarding its water permeability, and fouling potential. The optimum nanoclay concentration for permeability enhancement was different for each polymer, 1.5%, 2.0%, and 6.0% for PS, PES, and PVDF, respectively. This difference can be attributed to the differences of polymer’s hydrophobicity, based on the contact angle of a sessile water drop, which is higher for PVDF (PVDF is more hydrophobic than PS and PES). The flow improvement changed based on the main polymer. Significant changes in internal pore structure were observed for all membranes. The proportion of macrovoids was decreased and pores had a better connectivity across the cross section for PES and PS membranes. For PVDF membranes, the addition of nanoclay had a different effect on their microstructure. In this case, internal pores were 20% wider, factor that increased the average membrane porosity. The simultaneous evaluation of the clay nanoparticles used as an additive have clearly demonstrated its potential application for composite membrane production. It is also worth to note that the best way for identifying and evaluating the potential for an additive for membrane casting is considering its effects for different polymers, under the same casting conditions.

Effects of 1-methyl-2-pyrrolidinone (NMP) on polyamide-polysulfone TFC membrane pore morphology and ICP and membrane performance in forward osmosis

In forward osmosis (FO), the influence of the pore morphology in the membrane support layer on membrane performance is still unclear. As a result, investigation into the relationship between membrane pore morphology and membrane performance in FO is meaningful. During the fabrication of a thin-film composite (TFC) FO membrane, the membrane pore morphology is related to the composition of the coagulant bath. In this study, varied concentrations (from 0 to 9 wt.%) of 1-methyl-2-pyrrolidinone (NMP) in a coagulant bath were applied to fabricate TFC FO membranes of different pore morphologies. The morphologies of the fabricated membranes were then characterized using FESEM, and the performance of each of the membranes was evaluated using a lab-scale FO setup. The FESEM images indicated that the increase in the NMP concentration elongated finger-like pores and suppressed the formation of large pores in the profile. When the NMP concentration was 9 wt.%, the finger-like pores nearly spanned the entire SL of the fabricated membrane. In addition, the open pores at the membrane bottom surface became smaller and fewer in number with the increase in the NMP concentration. The experimental FO results of the membranes indicated that internal concentration polarization reduction could be achieved by creating more long finger-like pores in the membrane SL as well as more and larger open pores at the membrane bottom surface. All of the fabricated membranes had a low rate of water flux (Jw) to reverse salt flux (Js). In addition, the membrane fabricated with 3 wt.% NMP had the best performance in minimizing the draw solute loss, with a high Jw/Js of 4.2 L/g.

Effect of membrane pore morphology on microfiltration organic fouling: PTFE/PVDF blend membranes compared with PVDF membranes

This study provides an insight into the effect of membrane pore morphology on microfiltration organic fouling, by comparing the fouling behaviors of a series of polytetrafluoroethylene/polyvinylidene fluoride (PTFE/PVDF) blend membranes with those of PVDF membranes at varied hydrophobicity and pore size levels. Unlike the PVDF membrane pore morphology which resembled particulate bed, the PTFE/PVDF blend membrane morphology was characterized as fibrous network with thread-like pore walls and interconnected pore channels of high structural complexity. Fouling evolution rate and irreversibility of these membranes were assessed using batch filtration tests, with a membrane bioreactor supernatant employed as the foulant. Fouling of the PVDF membranes deteriorated significantly with increasing hydrophobicity and decreasing pore size. By comparison, all the blend membranes showed moderate fouling, much less sensitive to hydrophobicity and pore size. It was demonstrated that hydrophobicity and pore size pertain to hydrophobic adsorption and size exclusion respectively, which are fundamental membrane–foulant interactions for promoting fouling. Pore morphology may affect fouling by influencing these interactions. For the case of the blend membranes, it was inferred that the thread-like pore walls with limited adsorption area discouraged hydrophobic adsorption, while the high structural complexity encouraged size exclusion on the other hand.

Transport through composite membrane, part 1: Is there an optimal support membrane?

Abstract The effects of the support membrane pore size and porosity on diffusive transport through composite membranes have been investigated theoretically. Both 3D and 2D models were developed to mechanistically describe the relationship between support membrane pore structure, support material permeability, coating film thickness and the resulting composite membrane permeability; in addition, an analytical model was developed as an approximate, but more convenient approach for assessing trends of composite membrane transport. Model results suggest the choice of support is increasingly important as thin film permeability increases (i.e., less dense or thinner). For fixed coating film permeability, changes to support membrane pore structure create practically important changes to observed water flux and salt rejection by nanofiltration, brackish water reverse osmosis (RO), and seawater RO membranes. The diffusivity of the permeating species in the microporous phase of the support material may also contribute to the overall permeability of the composite membrane. Finally, a systematic numerical study suggests, for the first time, that the local permeate water flux through composite membranes is dictated by support membrane pore morphology, creating localized high flux “hot spots” with potentially high fouling and scaling propensity.

Synthesis of controlled thermoresponsive PET track‐etched membranes by ATRP method

AbstractControlled thermoresponsive PET track‐etched membranes were synthesized by grafting N‐isopropylacrylamide (NIPAAm) onto the membrane surface via atom transfer radical polymerization (ATRP). The initial measurements were made to determine the anchoring of ATRP initiator on PET membrane surface. Thereafter, polymerization was carried out to control the mass of polymer by controlling reaction time grafted from the membrane surface and, ATR‐FTIR, grafting degree measurements, water contact angle measurements, TGA, and SEM were used to characterize changes in the chemical functionality, surface and pore morphology of membranes as a result of modification. Water flux measurements were used to evaluate the thermoresponsive capacity of grafted membranes. The results show the grafted PET track‐etched membranes exhibit rapid and reversible response of permeability to environmental temperature, and its permeability could be controlled by controlling polymerization time using ATRP method. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011.

Preparation of high-capacity, weak anion-exchange membranes for protein separations using surface-initiated atom transfer radical polymerization

This contribution describes a method to prepare high-capacity anion-exchange membranes for chromatographic bioseparations. Surface-initiated atom transfer radical polymerization was used to graft poly(2-dimethylaminoethyl methacrylate) (poly(DMAEMA)) nanolayers from the pore surfaces of commercially available regenerated cellulose membranes. Initial measurements were made to determine the thickness evolution of the poly(DMAEMA) nanolayers, using a model flat substrate designed to mimic the three-dimensional nature of initiator incorporation into the membrane. Thereafter, polymerization time was used as the independent variable to control the mass of polymer grafted from the membrane surfaces and, thus, the protein binding capacity. ATR-FTIR, AFM, and SEM were used to characterize changes in the chemical functionality, surface topography, and pore morphology of membranes as a result of modification. Bovine serum albumin was used to evaluate the static protein binding capacity of poly(DMAEMA)-modified membranes. Maximum static binding capacities increased with increasing polymerization time in a linear fashion for short polymerization times (<6 h). For longer polymerization times, capacity increased non-linearly, eventually reaching a plateau value of 66.3 mg/mL.